Formalin injection in the tail facilitates hindpaw withdrawal reflexes induced by thermal stimulation in the rat: effect of paracetamol.

With the aim to develop a simple behavioural method for the study of hyperalgesic processes and for the evaluation of anti-hyperalgesic properties of analgesic drugs, the effect of the tail injection of formalin (10% formaldehyde intradermally) on hindpaw nociceptive thresholds to thermal stimulation was evaluated in the rat. The formalin injection in the tail induced a significant reduction of plantar test latencies. The pretreatment with the competitive N-methyl-D-aspartate (NMDA) receptor antagonist CGP 37849 (2.0 mg/kg) completely blocked the hyperalgesic action of the formalin. The analgesic drug paracetamol (25 mg/kg) was able to prevent hyperalgesia. However, this drug was unable to block hyperalgesia when already established. Our results suggest that this method could be used for the evaluation of analgesic drugs in an experimental setting representative of clinical pain.